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Sampling and analyzing emulsions
Samples of an oil emulsion may be required for several reasons, including crude specification verification, performance evaluation of the emulsion treating system, or simply, laboratory testing.
Invariably, the emulsion to be sampled is under pressure, and special procedures must be used to obtain representative samples. For crude specification testing, it is not important to maintain the integrity of the water droplets; however, the sample location point may be critical. In general, samples should not be withdrawn from the bottom of the pipe or vessel. Free water may be present and accumulate at the bottom of the pipe or vessel, affecting the basic sediment and water (BS&W) reading. Neither should the sample be withdrawn from the top of the vessel because it primarily will be oil. The best position in the pipe to take an emulsion sample is on the side, preferably with a quill. Turbulence and high fluid velocity in the pipe may also ensure that the sample is homogenous and representative.
Emulsion samples should be representative of the liquid from which they are taken, so emulsification should not be allowed to occur when the sample is extracted. This is especially true of liquids under pressure. For example, for samples obtained at the wellhead, manifold, or oil and gas separator, emulsification can occur because of the turbulence created while the sample is removed from the pressure zone to the sample container. Although such a sample might show a high percentage of emulsion, the oil and water in the system actually might not be emulsified.
Sampling procedure for pressurized sources
A special procedure is used to obtain representative samples from pressurized sources without further emulsification of the liquids. Fig. 1 shows a floating piston cylinder used in the procedure. The cylinder is first evacuated and filled with a pressurizing fluid (for example, glycol or a synthetic oil) on one side of the floating piston. The top of the cylinder (evacuated side) is then connected to the sampling location from which the sample is to be taken. The bottom valve on the cylinder is closed and the top valve slowly opened to pressurize the fluid in the cylinder. This is usually a small amount because of the low compressibility of the liquid in the cylinder. Once the top valve is completely open, the bottom valve is opened very slowly to drain the pressurizing liquid while allowing the sample liquid to be taken in from the top into the cylinder. The procedure should be performed slowly to obtain the sample without a pressure drop between the cylinder and the sampling location. Another variation of the method is to charge the sample into a simple cylinder, without a floating piston, filled with water or mercury. Once the sample is captured, the cylinder can be depressurized extremely slowly with little effect on the sample. In situations in which this procedure is not possible, the best sampling method is to bleed the sample line very slowly into the sample container. The idea is to minimize shear and reduce emulsification that may be caused by the sampling procedures.
Pressure zone samples
Samples from a pressure zone can be taken without further emulsification of the liquids if the velocity of the discharging liquid is controlled. One method is to use a piece of small-diameter tubing that is 10 to 15 ft long. One end of the tubing is connected to a bleeder valve on the line or vessel from which the sample is to be extracted, and the other end is connected to the sample container. The bleeder valve is opened fully, and the sample is allowed to flow through the small-diameter tubing into the container. Emulsification caused by pressure differential may be largely eliminated by flow through small-diameter tubing; however, contact with the tubing walls might produce coalescence, or perturbations to the flow caused by the passage of solids or large water drops might produce emulsification.
Another method for withdrawing representative emulsion samples is to use a sample container that initially is filled with water and is equipped with valves at the top and bottom, with the top valve connected to the point from which the sample is to be extracted. The top valve of the container is opened first, and the container is pressured from the line. The valve at the bottom of the container then is opened, and the water is discharged into the atmosphere as the sample enters the container. No emulsification will occur in the container because there is no pressure drop between the source and sample container to cause turbulence. After the sample has been taken, pressure can be bled off through a third valve with little effect on the sample.
Determining BS&W content
The BS&W content of crude oil is determined using small centrifuges that are driven by hand or by electric motor. A small measured volume of sample is diluted with solvent and placed in graduated glass containers that then are inserted into the centrifuge and spun for a few minutes at speeds of 2,000 to 4,000 rev/min. The oil, water, and solids are separated by centrifugal force, and the percentages of each can be read directly from the graduated containers in which the sample is centrifuged.
Standards for testing and measurement
These methods are described in the American Standards for Testing and Measurement publication ASTM D-96 for field measurements and ASTM D-4007 for laboratory procedures. Methods for taking and analyzing samples of crude oil for custody transfer are included in the API Manual of Petroleum Measurement Standards.
Emulsion separation index test
The emulsion separation index (ESI) test provides a measure of the stability of the emulsion. This emulsion bottle test is a quantitative method for demulsifier testing and involves the following procedure.
- The crude oil emulsion sample is tested as soon as possible after it is received in the lab. The pressurized method for sampling the emulsion is recommended. The samples are remixed with a standard bottle shaker for approximately a minute. The same amount of shaking should be used in all tests.
- The mixed emulsion sample is added to 100-ml standard centrifuge tubes.
- The centrifuge tubes are placed in a water bath for a minimum of 30 minutes to reach the desired temperature.
- The required dosage of the chemical is added to the centrifuge tubes. The amount of chemical is based on the total amount of emulsion (oil and water).
- The tubes are shaken by hand a given number of times (approximately 20 shakes) and placed in the water bath at the desired temperature.
- The amount of water separated is measured with time (5, 10, 15, and 20 minutes).
- After 20 minutes, the tubes are centrifuged for another 20 minutes at the desired temperature, and the final amounts of water and emulsion or rag layer are measured.
- Generally, these experiments should be done in sets to investigate the effect of certain variables. All efforts should be made to keep all the variables constant except the one under investigation.
The ESI is then calculated from the measured oil/water separation data.
where I = emulsion separation index, W = water separation at a given demulsifier concentration/time as a percentage of BS&W, and n = number of experiments.
- Direct comparison of demulsifier dosage obtained in the laboratory with field observations should always be made with caution because the laboratory experiments are made under static conditions, and field usage observations are made under dynamic conditions. Furthermore, ESI tests are done on dead crude, while field usage tests are on live (gas dissolved) crudes. However, laboratory testing is excellent for screening wellhead samples for relative emulsion tightness; evaluating prospective demulsifiers; and evaluating the effects of different variables on emulsion resolution because all the conditions are kept constant except the variable under investigation.
- Although ESI quantifies the bottle tests, it has a qualitative edge and has a range of reproducibilities because of several possible errors. These possible errors include sampling error, operator error, inability to read the level of water separated properly because the oil has a tendency to stick to the glass, and temperature error.
- There is a definite effect of aging. The longer an emulsion (oil/water mixture) stays in the lab before testing, the higher the demulsifier dosage required to break it; therefore, for best results, ESI tests should be done on fresh emulsion samples.
Given the data in Tables 1 and 2, calculate the ESI for demulsifier C.
Solution. Eq. 1 can be used to give
|I||=||emulsion separation index|
|W||=||water separation at a given demulsifier concentration/time as a percentage of BS&W|
|n||=||number of experiments|
- ASTM D-96-88 (withdrawn 2000), Standard Test Methods for Water and Sediment in Crude Oil by Centrifuge Method (Field Procedure). 1988. West Conshohocken, Pennsylvania: ASTM.
- ASTM D-4007-02, Standard Test Methods for Water and Sediment in Crude Oil by Centrifuge Method (Laboratory Procedure). 2002. West Conshohocken, Pennsylvania: ASTM.
- Manual of Petroleum Measurement Standards, API Manual. 2004. Washington, DC: API.
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